Solid gas sorption systems are used to produce cooling and/or heating by repeatedly desorbing and absorbing the gas on a coordinative complex compound formed by absorbing a polar gas refrigerant on a metal salt in a sorption reaction sometimes referred to as chemisorption. Complex compounds incorporating ammonia as the polar gaseous refrigerant are especially advantageous because of their capacity for absorbing large amounts of the refrigerant, often up to 80% of the absorbent dry weight. The complex compounds also exhibit vapor pressure independent of the refrigerant concentration and can be made to absorb and desorb very rapidly. Apparatus using complex compounds to produce cooling are disclosed, for example, in U.S. Pat. Nos. 5,161,389, 5,186,020, and 5,271,239. Improvements in achieving high reaction rates for the complex compounds are achieved by restricting the volumetric expansion of the complex compound formed during the absorption reaction of the gas on the metal salt. The methods and apparatus for achieving such high reaction rates are disclosed in U.S. Pat. Nos. 5,298,231, 5,328,671, 5,384,101 and 5,441,716, the descriptions of which are incorporated herein by reference.
While increased reaction rates have resulted from the aforesaid methods, it has been determined that repeated and relatively long-term absorption and desorption cycling of the complex compounds, particularly those using ammonia as a refrigerant, leads to sorbent migration even in the confined reaction chamber. It has also been found that the sorbent migration increases as higher sorption rates are used. Such migration may lead to uneven sorbent densities which in turn cause force imbalances in the heat exchanger structure, often resulting in deformation of the heat transfer surfaces and/or internal structures. As the heat exchanger structure becomes modified or compromised, heat and mass transfer reductions occur as does the sorption rate of the process. As sorbent migration continues, significant losses in performance efficiency are realized as is the possibility of failure of the reactor especially where it is exposed to high reaction rate sorptions.
Although improvements in attempts to overcome sorbent migration have been made for metal hydrides, such procedures and techniques have not been found to be suitable for ammoniated complex compounds. In U.S. Pat. No. 4,507,263, there is described micro-immobilization for metal hydride using a sintering process in which a metal hydride powder is embedded in a finely divided metal and the mixture sintered in a furnace at 100xc2x0-200xc2x0 C. using hydrogen pressure of 250-300 atmospheres. Although such a process reportedly results in mechanical stability for metal hydrides even after 6,000 cycles, the process is not effective for ammoniated complex compounds which exhibit much larger forces as compared to those experienced with metal hydrides. For example, where ammoniated complex compounds are absorbed and/or desorbed above about 3 moles NH3/mole sorbent-hr, the forces exercised on a sintered metal structure are so large as to result in deformation of the structure. Moreover, for most practical applications using complex compound technology, practical life expectancy of the reactors will exceed 6,000 cycles by an order of magnitude.
In the present invention, a sorber heat exchanger, i.e., a reactor, is provided with a sorbent/substrate composition comprising a substrate material substantially inert to a polar gas and incorporating a salt of a metal on which the polar gas is to be absorbed or with a complex compound formed by absorbing the polar gas on the metal salt. The use of the sorbent/substrate composition results in substantial micro-immobilization of the solid sorbent. According to the invention, it has been found that for sorption processes in which repetitive absorption and/or desorption reactions in excess of 200 repetitions exceed, at least temporarily, reaction rates of 3 moles gas/mole sorbent-hr, sorbent migration is substantially reduced by utilizing a suitable substrate material on or in which complex compounds or metal salts have been incorporated. Reduced sorbent migration improves the performance of the apparatus as well as the sorption cycle capability and expected life of the apparatus. The aforesaid invention is also useful in metal hydride sorption systems in which hydrogen is alternately absorbed or desorbed, particularly if high reaction rates or multiple thousand reaction cycles are desired. Such improvements as well as a more detailed description of the invention are described hereinafter.